This invention relates to a color television camera in which the color component signals representing the first and second colors of a color image are reproduced utilizing the difference of modulation phase relationships of the color component signals, at least one of which changes in successive scan lines.
A color television camera of the type employing a single pick-up tube for producing a color video signal by processing successive line signals produced by line scans is shown, for instance, in the U.S. Pat. No. 3,647,943 or Japanese Published Pat. No. 45-8699. In those cameras, the first color of the color image, such as red, is spatially modulated by a first striped color filter so as to have a first phase relationship, such as an in phase relationship, in successive scan lines, and the second color of the color image, such as blue, is spatially modulated by a second striped color filter so as to have a second phase relationship, such as an opposite phase relationship, in successive scan lines. As well known in the art, such a first striped filter may be a W-Ye striped filter disposed perpendicular to the direction of horizontal scanning and containing a plurality of striped filter element pairs, W-elements which are transparent, and Ye-elements which pass red and green light. The second filter may be a W-Cy striped filter disposed at a different angle from the W-Ye striped filter and containing a plurality of striped filter elements, W-elements, and Cy-elements which pass blue and green light.
A modulated component of the composite signal produced by line scans of the pick-up tube is provided to a comb-filter comprising a delay line (1H-delay line) which has a delay time of one horizontal scanning period, an adder and a subtractor, such as shown in FIG. 1, to recover the first and second color component signals. Then the color component signals are combined with a luminance signal derived from the composite signal to produce color difference signals.
In such a color television camera, when the video pattern of the color image has no relationship or even a partial relationship, such as a bar pattern including the vertical edge portions therein, in successive lines of scan, such as shown in FIG. 2A, color errors take place at the edge portions in the reproduced color image on the picture screen of the color television reproducing apparatus. The color error is especially conspicuous when the optical image includes black and white patterns.
Supposing that the n-th horizontal line scan occurs on a block area with a reflectivity of 10% and the (n+1)-th horizontal line scan on a white area with a reflectivity of 90%, the modulated component signals derived from the pick-up tube are expressed by the following equations: EQU Fn=0.1.multidot.{R.multidot.sin .omega.t+B.multidot.sin (.omega.t+.phi.)}(1) EQU Fn+1=0.9.multidot.{R.multidot.sin .omega.t+B sin (.omega.t+.pi.+.phi.)}(2)
where R and B denote the red and blue components respectively, and .phi.denotes a phase constant determined by the position of the line scans. These signals which appear at the input terminal and the output terminal, respectively, of the delay line (1H-delay line) are added to and substracted from each other in the comb-filter, to produce red component signal Fr and blue component signal F.sub.b, respectively, as follows: EQU Fr=0.5.multidot.R.multidot.sin .omega.t-0.4.multidot.B.multidot.sin (.omega.t+.phi.) EQU Fb=-0.4.multidot.R.multidot.sin .omega.t+0.5.multidot.B.multidot.sin (.omega.t+.phi.)
where each coefficient of R or B is converted so that it becomes unity when the video patterns scanned in the successive line scans are correlated to each other.
Therefore, if the ratio R:B equals 1:1, the amplitude of the detected color component signals R(t) and B(t) changes stepwise at such an edge portion, such as shown in FIG. 3. This change in level occurs because the color television camera employed in this system must combine the signals from two successive scans in order to produce the color component signals R(t) and B(t). The average level of the edge portion is half that of the correlated pattern. In contrast, in the luminance signal derived from a low pass filter from the output signal of the pick-up tube, such a change of the level does not occur even at the edge portion. This is because the luminance signal is derived from individual scans without requiring a combination of successive scans. In the television camera employed in this system the color difference signals are produced from the color component signals R(t) and B(t) and the luminance signal Y(t). When two successive scans are uncorrelated or only partially correlated the color component signals R(t) and B(t) produced from those successive scans are based on an artificially averaged video pattern of the two successive scans differing from the real video pattern of either of the two successive scans. As the result, a color error component is carried into the color difference signals at the edge portion of the optical image pattern.
There has been proposed an apparatus for suppressing such color errors in U.S. Pat. No. 4,104,679 issued to Kitamura, one of whose joint inventors is a joint inventor of the present invention. According to the apparatus proposed in U.S. Pat. No. 4,109,679, edge portions are detected by processing the luminance signal, and the detection signal is utilized to control the wave shape of the luminance signal used to reproduce the color signals, the wave shapes of the color component signals derived by the comb-filter, or both. This apparatus is effective when the signal ration R:B is about 1:1 and .phi. is nearly constant. However, the ratio is not always 1:1, but varies with the color temperature of the light source, the characteristics of each pick-up tube and also with the kind of pick-up tube. Moreover it is difficult to maintain .phi. constant due to scanning distortion of the vertical deflection device.